DepositionMechanismsinLayer-by-LayerorStep-by-Step...

International Scholarly Research NetworkISRN Materials ScienceVolume 2012, Article ID 701695, 13 pagesdoi:10.5402/2012/701695

Review Article

Deposition Mechanisms in Layer-by-Layer or Step-by-StepDeposition Methods: From Elastic and Impermeable Films toSoft Membranes with Ion Exchange Properties

Marc Michel, Valerie Toniazzo, David Ruch, and Vincent Ball

Advanced Materials and Struct Department, Public Research Center Henri Tudor, 66 Rue de Luxembourg,4002 Esch-sur-Alzette, Luxembourg

Correspondence should be addressed to Vincent Ball, [email protected]

Received 28 December 2011; Accepted 16 January 2012

Academic Editors: M. Padmanaban and H. Saxen

Copyright © 2012 Marc Michel et al. This is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

The modification of solid-liquid interfaces with polyelectrolyte multilayer films appears as a versatile tool to confer newfunctionalities to surfaces in environmentally friendly conditions. Indeed such films are deposited by alternate dipping of thesubstrates in aqueous solutions containing the interacting species or spraying these solutions on the surface of the substrate. Spincoating is more and more used to produce similar films. The aim of this short review article is to provide an unifying picture aboutthe deposition mechanisms of polyelectrolyte multilayer films. Often those films are described as growing either in a linear or ina supralinear growth regime with the number of deposited “layer pairs”. The growth regime of PEM films can be controlled byoperational parameters like the temperature or the ionic strength of the used solutions. The control over the growth regime of thefilms as a function of the number of deposition steps allows to control their functional properties: either hard and impermeablefilms in the case of linear growth or soft and permeable films in the case of supralinear growth. Such different properties can beobtained with a given combination of interacting species by changing the operational parameters during the film deposition.

1. Introduction

The functionalization of surfaces is of interest since theappearance of technology in the antiquity. More and moresurface coatings, as painting, were developed not only tochange the aesthetic appearance of a material but also toprotect it against the environment (for instance againstoxidants) and more and more to provide it with an additionalfunctionality, for instance drug release. This last point is ofparticular importance in the functionalization of biomateri-als.

The need for a better control of surface chemistry and theunderstanding that the fundamental chemical unit is eitheran atom or a stable assembly thereof, that is, a molecule,incited researchers to deposit single molecular layers atsurfaces. Hence, during almost the half first of the 20thcentury; the investigation of polymer and colloid depositionwas restricted to the deposition of monolayers.

The Langmuir or Langmuir-Blodgett deposition method[1], consisting in the transfer of amphiphilic molecules fromthe water-air interface, to a solid-air interface allows forthe transfer of multiple layers. But to reach that goal, itrequires very clean substrates and a dust-free atmosphere.In addition the multilayers of amphiphiles are not robustfrom a mechanical point of view and the deposition processis very slow. Some strategies have been developed to crosslinksuch assemblies using unsaturated molecules and UV light toinduce intermolecular bonds.

The first layer-by-layer deposition experiments from col-loids (assemblies having an hydrodynamic radius between1 nm and about 1 μm) were performed using oppositelycharged silica particles [2]. The buildup of such assembliescould be followed by means of an optical microscope. Thepossibility to obtain multilayered architectures of smallerassemblies was then totally neglected during almost 35 years.

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The need to obtain alternative deposition strategies to over-come the shortcomings of multilayered films obtained bythe Langmuir or the Langmuir-Blodgett deposition methodsled the researchers at the Institute for Physical Chemistryin Mainz to obtain layered architectures from polymers andbolaamphiphiles [3]. Very rapidly this concept was adaptedfor the alternated adsorption of oppositely charged polymers[4]. Such polymers are called polyelectrolytes. One has todistinguish between strong and weak polyelectrolytes. Theformer one carry a surface charge density (average chargeper unit length of the polymer) that is independent fromthe pH of the aqueous solution whereas the ionizationof the latter one is pH dependent. Since the first papersdescribing such architectures obtained trough sequentialdeposition steps have been published by Decher et al. in 1991,a fantastic enthusiasm for such coatings has appeared inthe scientific community. This deposition method will leadto coatings produced in a “Layer-by-layer” (LBL) manner.Nowadays, at the beginning of 2012, more than 1000scientific papers are published a year, tens of patents havebeen deposited and some commercial products based on theLBL technology have been pushed on the market. To cite onlya few: coatings for contact lenses (Ciba Vision), coatings forchromatography columns (Agilent Technologies).

The success of this coating technology relies on its sim-plicity and versatility as it can be easily automated (Figure 1)and, since that the LBL coatings can be deposited not only onplanar substrates but also on colloids, nanoparticles [5] andin the pores [6] of all these materials.

It is the aim of this small review to describe the fun-damental mechanisms allowing for the deposition of suchfilms and to provide a unifying view of this field in whicheach combination of polyelectrolytes is often treated byitself without comparison with other systems. The mainmessage of this paper is to show that films produced fromthe alternated deposition of mutually interacting species canhave properties similar to those of rigid and impermeablematerials or to those of highly permeable gels. In addition,such multilayered films display a dynamic response toexternal stimulus like a change in ionic strength or in theenvironmental temperature. Finally, some major applicationfields of LBL films will be described. It is not our aim to beexhaustive, but to provide a comprehensive overview of the”LBL” deposition research. Interested readers may find somespecialized and exhaustive review articles [7, 8].

2. Kinds of Molecules That Can Be Deposited,Deposition Methods and Growth Regimes ofFilms Produced in an LBL Manner

If LBL films are produced from oppositely charged poly-electrolytes, one speaks about polyelectrolyte multilayerfilms (PEMs). It was rapidly demonstrated that the LBLdeposition methods do not only work with oppositelycharged polyelectrolytes, to yield PEM films, but also with allkind of multitopic molecules presenting mutually interactingbinding sites. Among such molecules, one can site thefollowing.

(i) Polymers carrying hydrogen donor and acceptormoieties [9–11].

The fundamental mechanisms allowing for the depo-sition of films prepared by the alternated adsorptionof hydrogen bond donors and acceptors have beenreviewed recently [12].

(ii) Stereoregular polymers carrying sites of oppositechirality [13].

(iii) Host-guest interactions [14].

(iv) Charge transfer interactions [15].

(v) Specific biorecognition interactions like those be-tween streptavidin and polymers carrying avidinmoieties [16].

(vi) π-π interactions between molecules carrying aro-matic cycles and carbon nanotubes [17].

(vii) Covalent interactions using, for instance, polymerscarrying azides which react through 1,3 cyclo-additions with polymers carrying alkynes, one of thepossible “click chemistry” based reactions [18].

The LBL deposition method can not only be applied topolymers but also on combinations of polymers and particlesor just with mutually interacting nanoparticles [19].

The huge versatility of the LBL deposition methodsappears even more when one considers the different pos-sibilities to deposit such films at solid liquid interfaces.During almost 10 years, almost all the films producedin an LBL manner have been deposited using alternateddipping of the substrate to be coated in solutions containingthe interacting species (for instance polylectrolytes carryingsurface charge densities of opposite sign, or hydrogen bonddonors and acceptors). Two adsorption steps have to beseparated by rinse steps with the solvent (most often wateror an aqueous electrolyte solution) in order not only toremove weakly adsorbed polyelectrolytes but also to avoidcross-contamination of the solutions containing the mol-ecules to be deposited. Indeed, if the polyelectrolytes interacton a surface, they also interact in their solution: it is a well-known phenomenon (even if a lot of research remains tobe performed in this field) that the mixture of oppositelycharged polyelectrolytes leads to phase separation, particu-larly in conditions where the ratio of the number of positiveand negative charges is close to one [20]. Later on, thisphase separation phenomenon and its relationship with thefundamental understanding of the interactions at the originof the deposition of PEM films will be explained.

Several successful trials have been made to avoid theintermediate rinsing steps in order to increase the depositionspeed of PEM films. Among such methods one can note the“dewetting” method [21].

The alternated dip coating method is easy to implementand can be robotized in an straightforward manner. It ishowever time consuming since each adsorption and rinsingstep usually lasts over several minutes. Indeed molecules tobe deposited have to diffuse at the interface, to adsorb, andto find their equilibrium conformation, a process that maybe extremely slow.

ISRN Materials Science 3

2.6

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Th

ickn

ess

per

laye

r pa

ir/n

m

CNaCl0.1/(mol·L−1) 0.1

Figure 1: Evolution of the thickness increment per layer pairfor PEI-(PSS-PAH)n films deposited on glass substrates from aTris (Tris(hydroxymethyl)aminomethane) buffer at pH = 7.4 inthe presence of NaCl at different concentrations. The data havebeen plotted as the film thickness increment versus the NaClconcentration at a power α, with α = 0.10. The film thickness hasbeen measured by means of scanning angle reflectometry. Plottedwith the data from [29].

To speed up the deposition process, alternated spraydeposition [22, 23] and spin-coating [24] have been foundas good alternatives allowing for the deposition of a “layer”in a typical time of a few seconds. Simultaneous spraydeposition from two spray cans containing the interactingmolecules allows also to produce some coatings whosethickness increases linearly with the deposition time [25].Even if such films may have many similar properties to filmsproduced in a regular LBL manner, one cannot anymorespeak about “LBL deposition”.

Some investigations have shown that the nature of thedeposition method may have a pronounced influence of theproperties of the obtained coatings [26, 27]. It appears thatspin coated films are thinner, more transparent and of higherelasticity than their counterparts produced via alternateddipping [26]. This may be explained by the influence of shearforces applied during the spin coating on the conformationof the adsorbing polymer chains (see Scheme 1).

The LBL deposition method allows to deposit PEM filmson almost all kinds of surfaces irrespective of its chemicalcomposition and surface charge: indeed only the thicknessof the first few deposited layers is dependent on the usedsubstrate. As soon as the surface coverage of the substratereaches a critical level, the further deposition of polyelec-trolytes is not influenced anymore by the surface chemistryof the substrate.

The advantage of the LBL deposition method is that thefilm growth rate can be well controlled at the nanometerscale. In most cases the film growth rate is a linear functionof the number of the layer pairs deposited, one layer pairconsisting in the successive adsorption of the two interactingpartners (the same concept holds for LBL films build frompolymers interacting through hydrogen bonds or the otherpossible modes of interactions previously described). Thesefilms will be denoted by (A-B)n in the following where A andB are the interacting species and n is the number of depo-sition cycles, commonly called the number of “layer pairs.”The thickness increase corresponding to the deposition ofone “layer pair” is of the order of a few nanometers andcorresponds roughly to the sum of the characteristic size ofthe polycation and the polyanion. These sizes can be changedby modifying the conformation of the polyelectrolytes insolution. This is possible by playing on the charge densityof the polyelectrolytes, that is, by changing the pH of thesolution in the case of weak polyelectrolytes or by modifyingthe intramolecular interactions in a polyelectrolyte chainby screening the electrostatic interactions. This is easilyachieved by playing with the salt concentration. In thelow salt concentration regime, the film thickness incrementper “layer pair”, d, increases with the salt concentration(Figure 1) according to a power law of the type:

d ∼ d0

(C

C0

)α, (1)

where d0 is a characteristic film thickness, C the saltconcentration used during the experiment, andC0 = 1 mol/L.

For all the investigated polyelectrolyte pairs, the exponentα lies between 0.1 and 0.5 [28]. In the particular case of thefilms prepared from poly(allylamine hydrochloride) (PAH)and from poly(sodium-4-styrene sulfonate) (PSS) at pH =7.4 and in the presence of NaCl, the exponent α is equal to0.10 ± 0.04 [29].

Very soon in the history of LBL deposition, it was foundthat the average surface potential (measured as the zetapotential) of PEM films is alternated after each depositionstep [29–31]. This finding is expected if the degree of in-terpenetration of the chains is not too high (we will dealwith this point later on), meaning that each new adsorbedpolyelectrolyte neutralizes almost all the charges of thepreviously adsorbed one just providing a small amount ofexcess charge through polymer loops or trains danglingin solution. This excess of surface charge constitutes thenthe driving force for further adsorption of an oppositelycharged polyelectrolyte. It occurs in almost all the PEM filmsinvestigated up to now (Figure 2).

Upon the deposition of the last polyelectrolyte the surfacecomposition change is also reflected by a change in itswettability for water. This is reflected by a regular change ofthe static water contact angle between two limiting values(Figure 3) [32, 33]. This has first been exemplified withfilms made from the alternated adsorption of PAH andpoly(sodium acrylate) (PAA) [33].

The regular reversal of the surface potential seemshowever not to be an absolute requirement for the deposition


Dip coating

1 2 3 4

Spray coating Spin coating

Alternated Simultaneous

(1) Polyanion

(2) Wash

(1) Polycation

(2) Wash

Slow depositionautomatizable

efficient

Fast depositionbut drainage

waste of polyelectrolytes

Fast depositionbut only for small

surface areas

A B C

Zone 1

Zone 2

Drainage

d (nw)

Distancenozzle wafer

D

dSubs

trat

e

10 cm

Silicon wafer

Polyanion−

Polycation +

P = 1 bar

P = 1 bar

. . .. . .

Scheme 1: Illustration of the 3 most common deposition technologies leading to LBL or LBL-like coatings. The lower line outlines the majoradvantages and drawbacks of each of them.

40

20

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−40

PSSPSS PSS PSS

PSS

Glass

0 1 2 3 4 5 6 7

PAH PAH PAH PAH PAH

PEI

Number of layer pairs

ζ/m

V

∗∗

n

n

n

SO3Na

N

NN

HN

HN

NH2

NH2

Figure 2: Zeta potential of a PEM film made from PAH and from PSS from a Tris(hydroxymethyl)aminomethane buffer at pH 7.4 containing5 mg·mL−1 of each polyelectrolyte. The glass substrate was first covered with a layer of poly(ethylene imine) (PEI) aimed to produce a stableanchoring layer. Results plotted from the data of reference [29]. The structure of the repeat unit of each employed polyelectrolyte is indicated.


PAHPAH

PAHPAHPAH

PAH

PAAPAA

PAA PAA PAA PAA

Glass

35

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00 2 4 6 8


Stat

ic a

dvan

cin

g co

nta

ct a

ngl

e/de

gree

s

Figure 3: Static contact angles of water droplets (1 μL in volume)measured on PEM films made from PAH and PAA as thepolyelectrolytes (dipping solutions at pH 2.5 containing 10−2 M ofpolyelectrolytes based on the repeat unit’s molecular mass). Resultsplotted from the data of [33].

of a film from oppositely charged polyelectrolytes [34]. Thisrecent finding made on deposits made from PAH as a poly-cation and from sodium polyphosphate (PSP) as a polyanionraises many questions about the driving force leading to PEMfilms.

In addition to the linear growth regime of the PEMfilms, two other film growth regimes have been observed: thesupralinear [35–37] and the unstable adsorption-desorptionregime [38]. The supralinear growth of PEM films offersthe advantage to yield very thick films (more than 1 μmafter less than 10 alternated deposition cycles when the filmsremain hydrated) compared to the characteristic size of thepolyelectrolytes (a few nanometers) in a small number ofdeposition steps.

Linearly growing films display some fuzzy structurationwith small interpenetration of the alternating chains ashas been demonstrated by means of small angle neutronscattering [39, 40]. In these experiments, perdeuteratedPSS chains, giving some scattering contrast with respect tohydrogenated chains, were deposited at regular levels duringthe film deposition. Even if some small intermixing occurs,the chains stay roughly at the position where they wereinitially deposited.

In strong contrast with linearly growing films, in thecase of supralinear growth (Figure 4), which is indeed mostoften an exponential growth (up to a critical film thicknesswhere the growth turns again in a linear regime but with anincrement per layer pair that can be of a few hundreds ofnm), the films are characterized by a high chain mobility notonly in the direction perpendicular to the film, but also in the

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17.5 nm/layer pairNumber of layer pairs


0 5 10 15 20 25

160

120

80

40

0

Th

ickn

ess/

nm

Th

ickn

ess/

nm

Figure 4: An example of exponential growth followed by atransition to linear growth for PEI-(PGA-PAH)n films deposited onsilicon by means of alternated spray deposition (5s per depositionand buffer rinse step). The inset shows the thickness increase of thefilm for the first 22 layer pairs. The following function has beenfitted to the data: d = 2 + 9.52× exp(0.13× n), where d (nm) and nare the film thickness and the number of “layer pairs,” respectively.The slope of the linear growth regime following the first exponentialgrowth is of 17.5 nm per “layer pair”. These data have been plottedfrom the data of reference [41].

plane of the film. Figure 4 displays the change in thicknessof films made by the alternated spraying of poly(-L-glutamicacid) (PGA) and PAH as a function of the number of layerpairs.

This chain mobility has been observed by means ofconfocal laser scanning microscopy (CLSM) combined withfluorescence recovery after photobleaching (FRAP) [37, 42,43]. It was proven that the diffusion of at least one of theconstituent chains in and out of the film can be at theorigin of the exponential growth [37]. Indeed, let us makethe assumption that the film is constituted by a fraction offixed chains establishing relatively strong interactions withthe oppositely charged polyelectrolyte. This assumption isreasonable, otherwise the films would never be stable. Imag-ine now that the film preparation ends with the adsorptionof the diffusing polyelectrolyte. A further assumption isthat the only species able to diffuse is the polycation (aswas observed experimentally for films made from poly(-L-lysine), PLL, whereas Hyaluronic acid, HA, is not diffusingthrough the whole thickness of the film [37]. The adsorptionof PLL from the solution stops when the intermolecularelectrostatic repulsions between the similar charged chainsovercome the attractive interactions with the oppositelycharged chains. The nonadsorbed chains in solution arethen removed upon rinsing with the electrolyte solution(water plus a small molecular mass electrolyte). The chemical


potential of these chains in solution drops to −∞ as theirconcentration drops to zero. The weakly bound polycationsshould then desorb from the film in the solution in order toreach again equilibrium. This diffusion is nevertheless sloweddown by the presence of an electrostatic barrier at the filmsolution interface (Scheme 2). When the film is then putin contact with the polyanion solution, which is assumednot to diffuse in the film, the polyanionic chains adsorbon the surface of the film. This leads to a neutralizationof the positive charges at the origin of the initial diffusion(see Figure 2). Hence the film/solution interface acquires anegative charge and the electrostatic barrier impeding thediffusion of the fraction of free polycationic chains fromthe film to the solution disappears. Hence the polycationicchains start to diffuse in the solution where they buildup complexes with the available polyanions. Provided theformed complexes adhere to the film, the growth of the filmcontinues up to the level where all the available polycationicreservoir has been used. If the structure of the film ishomogeneous, and if the contact time with the polyanioncontaining solution is sufficient, the amount of polycationicchains diffusing out of the film should be proportional tothe film thickness. Hence the increase in film thickness, dueto polycation-polyanion complexation at the film/solutioninterface, should be proportional to the film thickness. Thisleads naturally to a film thickness increasing exponentiallywith the number of deposition steps.

Of course when the film thickness becomes importantand when the contact time between the film and thesolution is not increased accordingly, there is not enoughtime for the polycations close to the film/substrate interfaceto diffuse towards the film/solution interface where inter-polyelectrolyte complexation and film growth occur. Henceat a certain level of film growth, only a constant part ofthe film should be affected by diffusion of free chains outof the film, leading again to a linear growth as shown inFigure 4. Note that the slope of the linear regime correspondsto the value of the derivative of the film thickness versus thenumber of deposited “layer pairs” at the end of the expo-nential regime. The notion of “layer pair” is relative here,since the exponentially growing films are totally intermixedin opposition to their linearly growing counterparts in whichsome stratification and memory of the deposition processremains, as already noted previously. We hence prefer tospeak about films produced in a “step-by-step” (SBS) mannerto avoid the confusion the term “LBL films” could bring inthe reader’s mind.

More details about the mechanism of exponential filmgrowth can be found in [44].

Even if the mechanism described above seems reasonableand is partially confirmed by means of CLSM experimentsfor films made with fluorescently labelled polyelectrolytes,some other explanations have been proposed in the literature[45]. In addition, the model based on the diffusion of chainsthrough the whole film thickness relies on the fact thatthe films are homogeneous in the direction perpendicularto their average plane. There is plenty of experimental

evidence that both the linearly as well as the exponentiallygrowing films [46] have at least a 3-zone structure: thefilms have different properties at the film/substrate andfilm/solution interfaces than in their central zone. It is hencepossible that the transition from the exponential to the lineargrowth regime (cf. Figure 4) may be associated to someinternal rearrangement of the film leading to a progressivemodification of the internal film structure with time. Someincrease in the film elasticity (as probed by means of colloidalprobe Atomic Force Microscopy) has indeed been found as afunction of the storage time for (PLL-HA)n films [47].

The polydispersity of the employed polyelectrolytes alsoplays an important role in the occurrence of exponentialgrowth [48]: the higher the polydispersity of the polyelec-trolyte the higher the tendency for exponential growth.

The major difference in structure between the linearlyand exponentially growing films is depicted in Scheme 3.

The adsorption-desorption regime, in which eachadsorbed polyelectrolyte is desorbed upon contact with thesolution containing the oppositely charged polyelectrolyte,can be viewed as a situation where the polycation-polyanioncomplexes do not adhere to the substrate.

The difference in growth regime allows also a change inthe properties of the obtained films.

The fundamental interactions allowing the deposition offilms produced in an SBS manner will be described. Focuswill be given on PEM films made from oppositely chargedpolyelectrolytes because it is for this kind of films thatmost of the mechanistic investigations have been performed.This will allow to understand the fundamental differencesbetween the three known growth regimes of PEM films.

3. Interactions and Driving ForcesAllowing for the Deposition of Filmsin a Step-by-Step Manner

It is natural to think that the driving forces leading toPEM films is the electrostatic interaction between oppositelycharged chains. However, electrostatic interactions betweenoppositely charged polyelectrolytes in solution as well asbetween a solubilised polyelectrolyte and an oppositelycharged polyelectrolyte already adsorbed are of very peculiarnature [51]. Indeed there is not only an enthalpic contri-bution due to the interactions between point charges onthe oppositely polyelectrolyte chains, but also an entropiccontribution due to chain dehydration, conformationalchanges, and release of counter ions. This last contributioncomes from the fact that the charges on the polyelectrolytechain are surrounded by counter ions from the aqueoussolution. These small ions have to be removed from the closevicinity of the polymer chain during the complexation oradsorption process which induces an increase in their degreesof freedom and hence in the entropy of the system. Usuallythe balance between enthalpic and entropic contributionschanges with the salt concentration of the solution [52, 53].At low salt concentration, enthalpic contributions dominate,whereas entropic contributions become predominant for an


Chemicalpotential

ChemicalpotentialSurface barrier

Solution

Film

Polycation

Direction perpendicular

to the film surface

Direction perpendicular

to the film surface

State of the film after the adsorption

of the polycation

State of the film during the adsorption

of the polyanion

Polyanion

Scheme 2: Representation of the energetic state of an exponentially growing PEM film in the direction perpendicular to the film plane fortwo different states of the film deposition sequence: after the deposition of the polycation (left column) and during the adsorption of thepolyanion (right column). This scheme is inspired by the theory developed in [44].

intermediate salt concentration. At higher salt concentra-tions, the screening of the electrostatic forces is so highthat the formation of polyelectrolyte complexes and PEMfilms is totally hindered. The salt concentration at whichthe transition from an enthalpy-driven complexation toan entropy-driven one is dependent on the nature of thepolyelectrolytes. Of course temperature plays a huge role insuch processes where enthalpy-entropy compensation playsa role. It has been found that SBS films deposited from PAHand PSS (as in Figure 1) which display a linear growth atambient temperature and in the presence of 1.0 M NaCl turnto exponential growth when the temperature is raised above80◦C [54]. Similarly at ambient temperature, the growthof the PEI-(PSS-PAH)n films turns to exponential whenthe concentration of the supporting electrolyte, NaCl, isincreased [55]. These findings show that the nature of thefilm growth regime does not only depend on the nature of theused polyelectrolytes, but also on the interactions betweenthem. Those can be modulated by external parameterslike the temperature, the salt concentration as well as theaddition of a cosolvent. This discussion shows that the natureof the growth regime of PEM films can be adjusted byplaying on external parameters for a given combination ofpolyelectrolytes.

Recent research shows that there is a strong relationshipbetween interpolyelectrolyte interactions in solution, leading

or not to phase separation, and the possibility or not toobtain PEM films using a step-by-step deposition strategy[50, 56]. If one considers the intensity of the scatteredlight from a polycation-polyanion mixture in conditionswhere the amount of anionic sites matches the amountof cationic ones, that is, in conditions optimal for phaseseparation, as a function of the salt concentration, it appearsthat the obtained curve is almost homothetic to the PEMthickness evolution with the same polyelectrolytes. Indeedtwo different curve shapes have been observed: either amonotonous increase or the occurrence of a maximum(Figure 5).

In the case where the intensity of scattered light andthe film thickness increase with the salt concentration ina monotonous manner, the film growth is pretty slow andmostly linear with the number of deposition steps. On theother hand, in the case where the intensity of scatteredlight and the thickness versus salt concentration display amaximum, the growth regime turns from linear at low saltconcentration to exponential in the region of the maximum.At very low salt concentration the growth behavior of the filmlooks often zig-zag like, typical of an adsorption-desorptionprocess [50].

Hence the outcome of PEM film deposition as well as aqualitative prediction of the growth regime can be predicted


d A(n

m)

Number of layer pairs Number of layer pairs

0 10 20 30 40 50 60 70

200

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100

50

0

(PAH-PSS)n, NaCl 0.15 M, pH = 7.4(PAH-PGA)n, NaCl 0.15 M, pH = 7.4

3.3 nmper layer

pair

1200

1000

800

600

400

200

0

0 2 4 6 8 10 12 14 16

−ΔF

3/3

(Hz)

Scheme 3: Representation of the two most significant deposition regimes of PEM films (the adsorption-desorption regime [38] is notrepresented here because it leads to almost no film deposition). Note that the only difference between the two growth curves is the natureof the polyanion: the strong polyelectrolyte PSS (left column) being replaced by the weak PGA (right column). A representation of thechain packing in each kind of film is represented below the growth curves, the polycations and polyanions being represented in red andblue, respectively. The deposition of the (PAH-PSS)n and (PAH-PGA)n films has been investigated by means of optical waveguide lightmodespectroscopy and quartz crystal microbalance, respectively [49].

on the basis of interpolyelectrolyte phase diagrams. This isvery helpful to help to speed up the design of SBS films inadvance, that is, before their deposition.

4. Selected Properties of Films Produced ina SBS Manner

The linearly and exponentially growing films display ratherclear cut different properties which make them interestingfor a broad range of possible applications.

Usually the linearly growing films display “internalcharge compensation,” this means that, with exception tothe film/substrate and film/solution interface, the amountof charges provided by the polycation exactly matches theamount of charges provided by the polyanion, meaning thatthe bulk of the film does not need to incorporate counterions to ensure its electrical neutrality [57]. These films arenevertheless hydrated [58, 59] and display some porosity atthe nanometer scale. This porosity has been quantified bymeans of NMR cryoporometry in the case of (PAH-PSS)nfilms [60]. In quite good agreement with the cryoporometryexperiments, the same films display some permeability forions having an ionic radius of the order of one nanome-ter [61]. (PSS-PAH)n [62] and (PDADMAC-PSS)n films(where PDADMAC stands for poly(diallyldimethyl ami-nomethane)) become however impermeable to multivalent

ions like hexacyanoferrate when the number of “layer pairs”is typically higher than 5.

Usually the linearly growing films behave like elasticsolids, with an elastic modulus progressively decreasing whenthe salt concentration of the solution in contact with thefilm increases [63, 64]. At the same time, the ion doping[65] of the film increases and the stratification of the layersprogressively disappears [66], making such films close tothose obtained during an exponential growth regime.

In opposition to linearly growing films, exponentiallygrowing ones are highly hydrated (up to 80–90% of waterin volume fraction) [67]. They are extrinsically charge-compensated which allows them to behave as ion exchangemembranes [68, 69]. From a mechanical point of view theyare close to liquids as has been shown by piezo-rheometry[70].

A global view of the film properties depending on thegrowth regime is represented in Scheme 4.

The properties of PEM films can hence be changed whichallows a great versatility in their applications as will be shownin the next paragraph.

5. Selected Applications of FilmsProduced in a SBS Manner

In the field of materials science, PEM films have been usedto develop electronic devices like diodes (films incorporation


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orba

nce

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NaCl concentration/MNaCl concentration/M

NaCl concentration/MNaCl concentration/M

PLL/PSS complexes PDADMAC/PSS complexes

(PDADMAC-PSS)n films

(PLL-PSS)n films

Phase diagram

Deposition ofPEM films

−ΔF

3/3

(Hz)

−ΔF

3/3

(Hz)

Figure 5: Relationship between polyelectrolyte complexation in solution (upper row: the interaction between polyelectrolytes in a 1 : 1stochiometry, on the basis of monomer units, is quantified by means of light scattering) and PEM film deposition (lower row: the filmdeposition is quantified by means of quartz crystal microbalance with dissipation monitoring from polyelectrolyte solutions at the sameconcentration as that used for the phase diagrams). Two different combinations of oppositely charged polyelectrolytes are represented:poly(L-lysine hydrobromide)/poly(sodium 4-styrene sulfonate) (PLL/PSS) and poly(diallyldimethyl ammonium chloride)/poly(sodium 4-styrene sulfonate) (PDADMAC/PSS) adapted from [50].

clays during the layering process) [71], as anticorrosioncoatings [72], as antireflective [73] (with self cleaningapplications) and superhydrophobic coatings [74]. Recently,PEM films incorporating clays or other nanoparticles havebeen described as excellent fire retardants on the surface ofpolymer films [75]. More and more PEM films are used asmembranes separating the anode and the cathode in fuel cells[76].

PEM films can also be used as sensing matrixes atopan electrochemical [77] or optical transducers. Enzymes canbe encapsulated in such films and remain active for longertime durations than in solution [78]. Many bioapplicationsof PEM films are reported and have been reviewed [79,80]. Of particular interest is the finding that a change inthe mechanical properties allows to fine tune not only thecellular adhesion [81], but also the level of nuclear expressionof the adherent cells [82].

6. Conclusions and Perspectives

Films produced in a step-by-step manner can be depositedon a plethora of different substrates in an robotized mannerusing either alternated dipcoating, spray deposition or spincoating. The fact that different kinds of polymers, interactingeither through electrostatic interactions, hydrogen bonding,or other kinds of weak or covalent interactions, offers a veryversatile surface functionalization method. One fascinatingaspect of films made from polyelectrolytes, PEM films, isthe very easy deposition method which strongly contrastswith the complexity of the underlying mechanisms. Thereremain many fundamental investigations to be performed tobetter understand the relationship between the structure ofthe monomer units, the molecular mass and its distribution,as well as the external parameters (nature of the substrate,nature of the used electrolyte [38, 83]) on the deposition


Water content

Presence of ions from theelectrolyte

Permeabiltity

Mechanical properties

Low (20–30% in volume

fraction)

Very small (intrinsic charge

compensation)

For water, small ions

(depending on their size and

valency) and small molecules

Elastic solids

Linear growth Exponential growth

High (60–80% in volume

fraction)

Usually high (extrinsic charge

compensation)

For water, large ions

(depending on the sign of

their charge and valency),

large molecules and even for

colloids

Liquids or weak gels

Scheme 4: Influence of the growth regime and of the structure of PEM films on their properties.

kinetics and the dynamic responsiveness of the films to exter-nal stimulus. Exchange processes between polyelectrolytes inthe films and in the solution may also occur [84–86] with theconsequence of a pronounced change in film compositionand morphology.

Even more important, for future large scale applications,is to reach homogeneity of PEM films on large scale sub-strates (typically in the square meter range).

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